Polymer electrolyte fuel cell stacks (often referred to as “PEFCs”) offer advantages over other fuel cell stacks (often referred to as “stacks”), including lower operating temperatures during power generating operation, higher output densities and higher long-term reliability. Therefore, they have been drawing attention as a fuel cell stack used for fuel cell cogeneration systems.
Polymer electrolyte fuel cell stacks cause direct conversion of energy created by an electrochemical reaction into electric energy, accompanied with heat generation. This electrochemical reaction occurs between a fuel gas and an oxidizing gas (e.g., air), the fuel gas being generated from reforming of a raw fuel (e.g., city gas) and, more particularly, between hydrogen contained in the fuel gas and oxygen contained in the oxidizing gas. Incidentally, polymer electrolyte fuel cell stacks include unit cells (often referred to as “cells”). Each unit cell is composed of a membrane electrode assembly (often referred to as “MEA”) that includes a polymer electrolyte membrane and a pair of gas diffusion electrodes sandwiching the polymer electrolyte membrane therebetween; a pair of gaskets; and a pair of conductive separators. Either one of the conductive separators has, at its main surface in contact with a gas diffusion electrode, a groove-shaped fuel gas flow passage for permitting the passage of the fuel gas. The other conductive separator has, at its main surface in contact with the other gas diffusion electrode, a groove-shaped oxidizing gas flow passage for permitting the passage of the oxidizing gas. A pair of gaskets are disposed in the peripheral portion of the membrane electrode assembly so as to be sandwiched between the pair of conductive separators, thereby forming the unit cell. A specified number of such unit cells are stacked to thereby form a polymer electrolyte fuel cell stack. As a technique for manufacture of unit cells provided in a polymer electrolyte fuel cell stack, a continuous membrane electrode assembly production method has been proposed, which provides improved assembling (see e.g., Patent Document 1).
FIG. 31 shows a process chart outlining, in a schematic manner, production steps of a membrane electrode assembly (i.e., a catalyst layer coating step and a diffusion layer integration step) disclosed in Patent Document 1.
As illustrated in FIG. 31, in the catalyst layer coating step 310 of the production of the membrane electrode assembly disclosed in Patent Document 1, catalyst layers 331 are applied to the upper and lower surfaces, respectively, of the polymer electrolyte membrane 330 by coating and then dried with hot rolls 380 thereby forming a catalyst-layer polymer-electrolyte-membrane assembly 332. In the diffusion layer integration step 320 of the production of the membrane electrode assembly, diffusion layers 333 are applied to the upper and lower surfaces, respectively, of the catalyst-layer polymer-electrolyte-membrane assembly 332 and then heated by hot rollers 390, so that the diffusion layers 333 are bonded to the catalyst layers 331 respectively. Such a continuous production method is adopted to thereby facilitate the fabrication of the membrane electrode assembly in the course of the production of a unit cell.
As a polymer electrolyte fuel cell stack configuration, the so-called “stacking type” is generally known, according to which a specified number of unit cells are linearly stacked and fastened together so that the adjacent membrane electrode assemblies are electrically serially connected. When fabricating the polymer electrolyte fuel cell stack having the above stacking type configuration, a pair of end plates are provided at both ends of the stack of unit cells to sandwich the stack, and the pair of end plates and the stacked unit cells are fastened together by specified fastening members. Therefore, the polymer electrolyte membrane of the membrane electrode assembly needs to be protected by a proper protecting means so as to withstand the pressure of the fastening and so as not to suffer from mechanical damage caused by wear or the like during long periods of use.
As an attempt to meet the need, a membrane electrode assembly configuration having a frame-shaped protective film attached to the polymer electrolyte membrane has been proposed (see, e.g., Patent Document 2).
FIG. 32 is a cross-sectional view schematically illustrating a configuration of a unit cell in a solid polymer electrolyte fuel cell stack disclosed in Patent Document 2.
As illustrated in FIG. 32, frame-shaped protective films 220 formed from a fluororesin-based sheet are disposed on the main surfaces, respectively, of a solid polymer electrolyte membrane 210 such that the inner peripheral portions of the protective films 220 are covered with electrodes 213 respectively. In addition, gaskets 212 are disposed such that each electrode 213 is enclosed by its associated gasket 212 with a gap 214 therebetween. In this way, each protective film 220 is securely held between the gasket 212/the electrode 213 and the solid polymer electrolyte membrane 210, thereby reinforcing the solid polymer electrolyte membrane 210 at the gap 214. Therefore, damage to the solid polymer electrolyte membrane 210 can be properly prevented without need for increasing the thickness of the solid polymer electrolyte membrane 210.    Patent Document 1: JP-A-2001-236971    Patent Document 2: JP-A-05-21077